The paper analyzes the amino acid sequence of the receptor-binding site of hemagglutinin (HA) in the variants of pandemic Influenza A/H1N1 swi from 18 patients with moderate (n = 1) and fatal (n = 17) forms of the disease in 2009. Nine samples contained asparaginic acid at position 222 of HA1 (D). This site exhibited mutations in 9 samples: D222G (n = 3), D222N (n = 3), and D222G/D222N (n = 3). In one patient with the moderate form of the disease, D222G mutation was revealed after the second passage in the developing chick embryos; this mutation was not found In the primary sample from the patient. The findings suggest the mutant variants of the virus start to circulate among the population, which requires, firstly, continuation of molecular virological monitoring of the pandemic situation and, secondly, further study of the impact of amino acid substitutions at the receptor-binding site of HA1 on the Increased virulence of influenza A virus.
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